The invention relates to an implantable stimulator including a depletable voltage source for supplying electrical energy, the voltage source having a terminal voltage which varies as a function of depletion of the voltage source; an output stage receiving electrical energy from the voltage source and including means for generating a stimulation voltage that is increased with respect to the terminal voltage of the voltage source and a stimulation energy storage device for storing the stimulation voltage; and a control device receiving operational energy from the voltage source and being operatively connected for controlling a stimulation operation of the stimulation energy storage device.
Implantable stimulators--among which heart pacemakers have attained the greatest importance, but among which implantable defibrillators as well as other muscle, nerve or bone stimulators for long term use also have been given increasing circulation have been increasingly developed in the past years, starting with the introduction of microprocessors at the end of the Seventies, into implements with a multitude of functions and extensive internal recognition and adaptation functions. Along with this went the increasing complexity and importance of their control for dependable functioning of the implements, so that the assurance of an uninterrupted supply of the digital control functions with sufficient operating voltage is a basic requirement for all such modern implements; see U.S. Pat. No. 4,448,197, for example, in this connection.
Such implements are usually battery-powered. Today, lithium batteries are practically exclusively employed as batteries, which provide a voltage of approximately 3 V, which is sufficient for the power supply of the control circuit(s) on the basis of integrated circuits--especially in CMOS technology--as well as for many stimulating tasks, and which have a large specific energy storage capability; see W. Greatbatch, "A New Pacemaking System Utilizing a Long-Life Lithium Cell"; Dig. 9th Intern. Conf. on Medical and Biological Eng., Melbourne, 1971.
However, like every electrochemical cell, lithium batteries also have a characteristic discharge curve, which is marked by a rise of the terminal resistance with increasing discharge, i.e. increasing length of use of the implanted implement. However, the maintenance of a dependable implement function requires the provision of stimulation pulses of a defined, constant voltage or those which, in many applications (for example with heart pacemakers) after a certain length of employment can be set even higher than the initial values. There are furthermore also applications--again especially with heart pacemakers, but particularly also with defibrillators or cardioverters--, in which the terminal voltage of even a fresh lithium battery is not sufficient for (dependably) meeting the stimulation task.
For these reasons heart pacemakers--as already proposed in the above mentioned publication of GREATBATCH and in more detail in U.S. Pat. No. 3,707,974--have output circuits with means for increasing (in particular multiplying) the output voltage in respect to the terminal voltage. In many cases these means are called "charge pump" or--after the main functional elements, namely switched capacitors--SC (switched capacitor) transformers, and usually include a so-called pump-capacitor, through which the energy is transferred from the battery into the output stage, and switching means for controlling the energy transfer assigned to it. An externally programmable charge pump operating on a digital basis with several pump capacitors and switches for realizing different output voltages is described in European Patent EP 0 000 983 B1. A modern arrangement of this type is described in U.S. Pat. No. 5,387,228, by means of which controlled or uncontrolled stimulation voltages can be made available as a function of the degree of battery exhaustion.
It is also known to equip heart pacemakers with means for indicating an extensive exhaustion of the battery--a so-called EOL (end-of-life) detection--, in order to assure the timely implantation of a fresh pacemaker before life-threatening losses of function occur.
An implantable stimulation device in accordance with the species is known from EP 0 463 410 B1, in which a lowest permissible value of the stimulation interval is preselected for a time dependent control of the pulse generator as a function of the degree of exhaustion of the battery.
The switch-over to a lower stimulation rate than that indicated by means of a physiological sensor, or from a mode of operation with high current use to one with lower current use when falling below a preselected battery voltage threshold, is proposed in EP 0 431 437 A2.
An implantable cardioverter is described in U.S. Pat. No. 4,548,209, wherein the time regime during the charge of the high voltage capacitor for making the cardioversion voltage (DC-DC conversion) available is controlled via an inductive coupling as a function of the voltage or the internal resistance of the battery.
With the known charge pumps the problem arises that their activation without paying attention to the degree of their exhaustion results in an uncontrolled drop of the terminal voltage, wherein there is a danger that this falls below the minimal voltage required for operating the control circuits. This can have very disadvantageous, possibly irreparable results for the function of the stimulator. The said problem can become relevant in actuality particularly, if it is necessary in the result of a stimulus threshold, to program an increased stimulation amplitude after an already extended length of operation of a pacemaker (i.e. with the battery already drained to a large extent).